The concurrence and three-tangle, which measure entanglement of two-qubit
and three-qubit states, have been known to be the absolute values of
hyperdeterminants for two and three qubits respectively [1]}.
Recently, polynomial invariants have been proposed to construct entanglement
monotones. The absolute values of the polynomial invariants obtained in [2]} may be used to construct entanglement measures of four-qubit
states. We expect that the absolute values of the polynomials in this paper
can be considered as entanglement measures.
